1. Field of the Invention
The present invention relates to an intake piping structure for a multi-cylinder engine.
2. Description of Related Art
It is known that the charging efficiency is enhanced by utilizing the inertia effect or the resonance effect of intake air within an air intake system in order to improve output performance of the engine for an automotive vehicle.
In charging by taking advantage of the inertia effect, on the one hand, when the engine is in a predetermined rotational region, or in a tuning rotational region, an intake negative pressure wave of intake air generated within the air intake port in association with opening an intake valve in an initial stage of an inspiration stroke of each cylinder is spread or travelled at sound speed toward the upstream side along the inside of an individual air intake passage and the negative pressure wave is reversed to positive pressure wave in a predetermined volume chamber. Then, the positive pressure wave is spread or travelled in the same pathway toward the downstream side so as to reach the same air intake port immediately before the intake valve is opened, thereby forcing the intake air into the combustion chamber by means of the positive pressure wave and enhancing the charging efficiency.
In charging by taking advantage of the resonance effect, on the other hand, plural cylinders of the engine are grouped into plural cylinder groups so as to allow each of the grouped cylinders to have an equal inspiration stroke with each other. The individual air intake passages of the plural cylinders of each cylinder group are united into one merged air intake passage (resonance intake passage) at their upstream ends, and the merged air intake passage is in its predetermined position provided with a pressure-reversible section consisting of a volume chamber. A pressure wave of intake air travelling back and forth between the pressure-reversible section and each cylinder is caused to exhibit resonance within the merged air intake passage by coinciding a phase of a basic intake pressure wave generated in the air intake port of each cylinder of the cylinder group in a tuning rotation range of the engine with a phase of a reflection pressure wave reversed in the pressure-reversible section, thereby allowing the resonance to generate a resonance pressure wave having a large amplitude due to a vibration of pressure generating mergedly within each cylinder. This resonance pressure wave forces the intake air into the combustion chamber of each cylinder, thereby enhancing the charging efficiency.
A surge tank is generally employed as a merging section in which the pressure wave of intake air is reversed by taking advantage of the inertia effect. The surge tank, however, presents the drawbacks that a distribution of intake air to each cylinder and a dynamic effect cannot be made uniform because the inner length between the upstream passage and each individual air intake passage or the length of each of the individual air intake passages themselves varies with cylinder.
In order to solve these drawbacks, Japanese Utility Model Laid-open (kokai) Publication No. 88,062/1985 proposes an intake piping structure for an engine. In this structure, a merging section on the upstream end of the individual air intake passage is formed as a space of a nearly truncated-conical shape. To a smaller-sized side end of the merging section is connected a downstream end of the merged air intake passage, while plural individual air intake passages are connected each to a larger-sized side end thereof. Further, openings at the upstream ends of the individual air intake passages are so disposed as to be in line symmetry with respect to the axial line passing through the center of the opening at the downstream end of the merged air intake passage. This structure allows the distance from the opening at the downstream end of the merged air intake passage to the opening at the upstream end of each of the individual air intake passages in the merging section to be substantially equal, thereby making the distribution of intake air for each cylinder uniform and reducing resistance to intake air by avoiding the rapid curvature of a flow passage of intake air. Further, as the opening at the upstream end of each individual air intake passage is disposed in a position close to each other, this arrangement allows each of the individual air intake passages to be employed as a volume chamber for the inertia effect within the other individual air intake passage, thereby minimizing and making the size of the merging section itself compact.
For a V-type engine, when the individual air intake passages are intersected with each other between the banks, a space between its left-hand and right-hand banks is rendered small, leading to poorness in service performance and making it difficult to enlarge a size of each passage. Further, for an inline engine, for instance, when a mechanical supercharger is disposed at the side of a cylinder head, it is impossible to lengthen the size of the individual air intake passage due to interference with a supercharger. Therefore, it is difficult to perform a good inertia effect of intake air in a low-speed region of the engine because limits are placed upon the length of each individual air intake passage.
Hence, in order to extend the length of the individual air intake passage by avoiding those problems, the structure of the engine may be constructed such that the individual air intake passage is extended over the cylinder head and connected to the merged air intake passage and the merged air intake passage is curved in such a U-shaped manner as extending in parallel to the individual air intake passage. This arrangement, however, increases the height of the engine because this structure causes all the individual air intake passages to be extended in a nearly parallel way over the cylinder head and the individual air intake passages and the merged air intake passage to be superposed.
Alternatively, the structure of the intake piping may be arranged such that the merged air intake passage for each cylinder group is curved toward one end side in the direction of disposition of the cylinders. This structure, however, causes the length of the merged air intake passage to differ from that of the other merged air intake passage, so that the distribution of intake air and the inertia effect are made different between the cylinder groups. Further, as this structure causes the merged air intake passages to be juxtaposed, the transverse width of the engine can be lengthened.